Traditionally, porous sack paper or paper for manufacturing carrier bags has been manufactured without filling agents. Strength properties are especially important, particularly in the case of sack paper, and fillers weaken the strength. On the other hand, using fillers would include the advantage of being able to reduce the amount of expensive chemical pulp fibre, while providing improved printability and runnability of a board machine, as the filler does not absorb as much water as the chemical pulp fibre. Typically, the strength of sack paper is achieved by refining the paper pulp, using chemical pulp fibres and by a suitable fibre orientation at the wire section. It is necessary for a sack made of sack paper to have porosity, so that air can quickly be removed from inside the sack when filling the same. If air is not quickly removed, the filling of the sack is considerably decelerated. Generally, the sacks are made of several sack papers, but it may also be necessary for a single-layer kraft paper to have porosity.
The porosity of a packaging material is mostly measured in Gurley values or, alternatively, by a Bendtsen apparatus. Gurley can be used to measure how long it takes for 100 cm3 of air to flow through a sample area of one square inch under a pressure of 1.22 kPa. Typically, the Gurley value of a highly-porous sack paper should be less than 10 seconds and, in some cases, as low as 5 seconds. Generally speaking, it is quite difficult to achieve the desired Gurley values that are required of sack paper, single-layer kraft paper or paper, which bags are made of.
Typically, the porosity is provided by a suitable selection and refining of fibre pulp. However, this calls for a decrease in the productivity of the machine, as more time is required for refining in the process and the energy consumption increases. Enhancing the refining of pulp also decelerates the removal of water at the wire section, which means that the paper web conveyed to the press section contains more moisture. The porosity can also be provided by specific chemicals, such as retention agents.
In some cases, the porosity required is provided by making a required number of pinholes in the finished sack paper before filling the sack. In that case, the pinholes are made mechanically in the sack. However, the mechanical treatment may damage the sack paper. Furthermore, this also requires extra processing time and energy expenses.
As stated above, a typical way of providing the required porosity of the sack paper is high-density refining. Due to the tearing strength, in particular, the fibre orientation is an essential matter, which should be taken into account at the wire section. Generally, starch is used to increase the strength of paper. In that case, starch is dosed into high-consistency pulp.
In the paper making process, there is a need to combine porosity, strength, printability, quicker water removal at the wire section and the replacement of the more expensive chemical pulp fibre with cheaper materials, such as filler agglomerates. Furthermore, it would be preferable to be able to reach the high porosity level without mechanically perforating the material.